[scan-build] Supprot relative 'file' in cdb.

Excluded folders in scan build is turned to absolute path before
comapre to 'file' in cdb. 'file' in cdb might be a path relative
to 'directory', so we need to turn it to absolute path before
comparison.

Patch by Yu Shan

Differential Revision: https://reviews.llvm.org/D90362